The present invention relates to an image recording apparatus, and in particular, to an image recording apparatus which is favorable for continuously conveying a continuous recording medium such as paper or a sheet-like plastic film.
Image recording apparatuses have been used for stably conveying continuous paper into an image recording part. For example, Japanese Patent Unexamined Publication No. 8-26542 discloses an image recording apparatus incorporating a tractor for conveying a sheet to the upstream side of a photosensitive drum for forming a toner image, in which the rollers for conveying the sheet at a speed equal to that of the tractor are provided between the photosensitive drum and the tractor so as to prevent the sheet from slipping off or jamming in the tractor part.
Further, Japanese Patent Unexamined Publication No. 4-251060 discloses a system incorporating a pair of paper feed rollers provided between a recording part and a roll-like sheet, for slacking the sheet upstream of the recording part so as to prevent variation of conveying load in the recording part while the sheet is conveyed.
Japanese Patent Unexamined Publication No. 4-166367 discloses a system in which a sheet from a paper roll is applied thereto with a back tension by means of a pair of rollers provided between a recording part and a sheet so as to wind the sheet onto a roller for removing curling therefrom in order to stably convey the sheet.
Further, Japanese Patent Unexamined Publication No. 60-171962 discloses a system in which a roller displaces in response to a variation in the tension of a sheet so as to uniformly maintain the tension of sheet in order to prevent occurrence of print shift in multi-color printing.
The conveyance of a recording medium (which will be herein below referred as "sheet") such as a continuous sheet, causes variation (vibration) in the speed in the direction of conveyance of the sheet due to the feeding action of a sheet supply part or the like.
Further, the supply of a continuous sheet by tractors possibly causes a crosswise shift in the vicinity of the center part thereof between tractors due to some play between tractor pins and feed holes.
In the case of a conventional monosurface mono-chromatic image recording apparatus, the distance between the tractors has been small so that the degree of the shift is low, and therefore no serious problems have been raised.
Accordingly, in the case of high quality printing, in particular, in the case of printing a color image or of double surface printing, a print shift in an imager transfer part and a shift or speed variation in the case of printing with more than two colors, cause the occurrence of a transferred color drift, that is, the speed variation and the shift reduce the printing quality.
Accordingly, color matching and alignment between the front and rear surfaces of a sheet have to be made, and in order to precisely position a sheet in each of a plurality of image recording parts, it is necessary to restrain such a sheet speed variation or keep such a shift to a minimum.
However, the above-mentioned Japanese Patent Unexamined Publication No. 8-26542 and Japanese Patent Unexamined Publication No. 4-251060 do not address the problems caused by the above-mentioned variation in the image recording apparatus.
Further, the image recording apparatus disclosed in Japanese Patent Unexamined Publication No. 4-166367 results in such a problem that a sheet shifts widthwise or meanders since a pair of rolls for applying a back-tension to the sheet in order to remedy curling of the sheet, press only opposite ends of the sheet so that the pressing force is not uniform widthwise direction thereof.
Japanese Patent Unexamined Publication No. 60-17926 discloses the image recording apparatus which is used in a printing device such as an offset press including a large-scale mechanism for preventing occurrence of a print shift.